The present invention relates to a head driving mechanism for displacing a serial print type recording head towards and away from a platen in a recording apparatus such as a facsimile apparatus, a printer or the like.
FIG. 1 shows a prior art serial print type thermal printer. The known thermal printer includes a thermal head 1, a head carriage 2 having the thermal head 1 secured thereto, a head slide shaft 4, an operating lever 5, an electromagnetic solenoid 6 having a plunger 7 and a platen 8. The head slide shaft 4 is formed with an axially extending projection 3 having a raillike shape. The head carriage 2 is slidably fitted around the head slide shaft 4 so as to be moved in a printing direction of an arrow a. The head carriage 2 is engaged with the head slide shaft 4 through the projection 3 so as to be rotated together with the head slide shaft 4. Meanwhile, the operating lever 5 is pivotally provided at one end of the head slide shaft 4. The plunger 7 of the electromagnetic solenoid 6 is attached to the operating lever 5 so as to pivot the operating lever 5. By retraction of the plunger 7 upon energization of the electromagnetic solenoid 6, the head carriage 2 is rotated through the head slide shaft 4 in a direction of an arrow b for spacing the thermal head 1 away from the platen 8.
On the other hand, a coiled spring 9 for urging the operating lever 5 to rotate in a direction of an arrow c opposite to the direction of the arrow b is attached to the operating lever 5. During de-energization of the electromagnetic solenoid 6, the head carriage 2 is rotated in the direction of the arrow c by an urging force of the coiled spring 9 through the head slide shaft 4 such that the thermal head 1 is pressed against the platen 8 through a recording paper 10, whereby thermal recording is performed on the recording paper 10 by heat generated by the thermal head 1. Meanwhile, a mechanism for displacing the thermal head 1 in the printing direction of the arrow a along the head slide shaft 4 is provided and includes a driving pulley 11, a driven pulley 12, a belt 13 wound around the driving pulley 11 and the driven pulley 12 and a stepping motor 14 coupled with the driving pulley 11 such that a portion of the belt 13 is attached to the head carriage 2. By feeding of the belt 13 upon intermittent drive of the stepping motor 14, the head carriage 2 is intermittently displaced in the printing direction of the arrow a, so that thermal recording is performed on the recording paper 10 by heat generating drive of the thermal head 1 which is held in pressing contact with the recording paper 10 through the head slide shaft 4 by the urging force of the coiled spring 9. Subsequently, when the thermal head 1 has reached a stroke end in the printing direction of the arrow a, the electromagnetic solenoid 6 is energized. Therefore, the plunger 7 is retracted against the urging force of the coiled spring 9 and thus, the thermal head 1 is disengaged from the platen 8. In this disengagement state in which the thermal head 1 is disengaged from the platen 8, the head carriage 2 is returned to a print start position through reverse rotation of the stepping motor 14.
However, the known head driving mechanism has the following serious drawbacks. Namely, since it is so arranged that an engagement operation of pressing the thermal head 1 against the platen 8 and a disengagement operation of disengaging the thermal head 1 from the platen 8 are performed by rotation of the head slide shaft 4, not only the head slide shaft 4 is required to be formed with the projection 3 but the head carriage 2 is required to be formed with an engageable slot for slidably receiving the projection 3. In order to not only slide the head carriage 2 smoothly but rotate the head carriage 2 without play, the head slide shaft 4 and the head carriage 2 having the above described complicated shapes are required to be machined with high dimensional accuracy and thus, machining cost of the head slide shaft 4 and the head carriage 2 rises, thereby resulting in increased manufacturing cost of the known head driving mechanism.
Meanwhile, since it is so arranged that the head slide shaft 4 for sliding the head carriage 2 is rotated, a rotational force for rotating the head slide shaft 4 is necessarily applied to an end portion of the head slide shaft 4 by the operating lever 5. As a result, a pressing force applied from the thermal head 1 to the recording paper 10 changes according to position of the head carriage 2 relative to the head slide shaft 4 and thus, printing quality is aggravated due to nonuniform printing pressure.